NEW YORK (GenomeWeb) – Researchers from Mirna Therapeutics this month published new animal data demonstrating the potential of combining two tumor-suppressing microRNA mimics for the treatment of non-small cell lung cancer (NSCLC).
The mimics are synthetic versions of miR-34a and let-7b. The former — formulated with Mirna Biotech's Smarticle liposomes and dubbed MRX34 — is already testing in a Phase I trial for liver and blood cancers, while the latter is under preclinical development for undisclosed oncological indications.
Amid the newly reported positive data, which appeared in Oncogene, the company is considering advancing the miRNA cocktail as its next clinical candidate, Mirna Director of Research Andreas Bader told Gene Silencing News this week. However, doing so will depend on the performance of MRX34.
"We will watch our MRX34 trial very closely to learn from that as much as we can before we put another compound in the clinic," Bader said.
But once Mirna has solid safety data in hand, he doesn't expect much delay in getting its second candidate into human testing, noting that MRX34 has already laid much of the regulatory and manufacturing groundwork for follow-on compounds using the Smarticle technology.
While it has initially focused on hepatocellular carcinoma and other cancers with liver involvement, and just recently expanded its MRX34 trial to including hematological malignancies, Mirna has long maintained an interest in lung cancer given the disease's high unmet medical need.
In 2008, for instance, Bader presented data, generated in collaboration with former Yale University researcher Frank Slack, who contributed to the Oncogene paper, linking let-7 to lung cancer via its regulation of RAS. He also showed data pointing to the miRNA's ability to slow tumor growth in mice.
And in 2011, scientists from Mirna and Yale published a paper showing that mimics of either let-7 or miR-34 could inhibit tumor growth in a transgenic mouse model of lung cancer. Now, those collaborators have extended that work to show that combining the two miRNAs can have significant therapeutic benefit, as well.
In the Oncogene study, the scientists first tested the effects of introducing either miR-34a or let-7b into seven different lung cancer cell lines, or both miRNAs in combination at half the dose each.
Cells receiving either of the miRNAs showed decreased proliferation, while the combination was at least as effective, and in some cases more effective, the researchers wrote in the paper. Meanwhile, using the two miRNAs together resulted in decreased levels of more key oncogenes than either individual miRNA.
Next, the team moved to in vivo testing, using a mouse model that Bader said is the most accurate representation of human NSCLC in terms of disease progression, as well as response and resistance to standard chemotherapies.
The animals received injections of either let-7b or miR-34a, or a combination of both, at a total dose level of 1 mg/kg. The treatments, which were delivered using a neutral lipid emulsion delivery agent that had been used in previous preclinical studies, were given once every other day for a total of 11 doses. The delivery agent alone was used as a control.
A reduction in tumor nodules was only apparent in animals receiving the combination therapy, but because many of them became too sick to withstand dosing or died, the statistical power of this experiment was reduced. As such, the team moved onto a larger study, this time using the Smarticle delivery technology, and dosed the mice earlier than before.
Animals were dosed similarly to the previous ones, then sacrificed either 24 or 48 hours after the final injections and evaluated for tumor burden, signs of toxicity, accumulation of miRNAs in whole blood and lungs, and relevant markers.
In line with previous studies, there was no evidence of whole organ toxicity or elevation in any serum cytokines. Animals receiving the combination therapy experienced a reduction in overall tumor burden, which was attributed to a decrease in individual tumor size.
"Notably, average lung tumor volumes from the dual treated animals were also significantly smaller than average volumes from either of the single miRNA-treated groups, suggesting that combinatorial treatment with let-7b and miR-34a has a significant effect on tumor growth," Bader and colleagues wrote in their paper.
An analysis of the tumors that remained in animals receiving the miRNAs confirmed that key oncogenic targets were downregulated with both the combinatorial and individual treatments.
Lastly, the investigators evaluated the effect of miRNA treatment on survival, administering 10 mg/kg doses of either let-7b or miR-34a, or half doses of both miRNAs together, three times a week for eight weeks.
At the end of the eight weeks, roughly half of the animals had died. However, there was a clear survival benefit for those receiving the combination therapy or miR-34a alone. Specifically, control animals had a median survival of 57 days following the first injection, and miR-34a treatment increased that to 81 days. The combination treatment improved survival to 78 days.
In the end, the data show that "a combination of two biologically relevant, tumor-suppressive miRNAs is superior in its ability to repress oncogene expression, preventing proliferation and invasion of cancer cells in culture, to inhibit tumor proliferation in vivo and to confer a survival advantage," the investigators stated in Oncogene. "Furthermore, it is expected that this miRNA combination, which hits multiple relevant biological targets, would prevent the onset of acquired resistance that has been observed with other therapies."
Although the combination treatment did not offer any survival benefit compared with miR-34a therapy alone, the team hypothesized that this may be due to the low levels of miR-34a at baseline in treated animals.
"When the miRNAs are supplemented, the change in miR-34a levels is much larger than that of let-7b, which does not change strongly," they noted. "It is possible that in this experimental system the net effect of the large change in miR-34a levels is driving the survival effect."
When it comes to treating humans, the combination therapy is likely to have increased efficacy by blocking secondary resistance, they added.